A hollow fiber type membrane filtration device used in a reverse osmosis, an ultrafiltration, a microfiltration, or the like is already utilized in various fields. With the expansion of the uses, a hollow fiber type filter which can withstand a rapid temperature change has been required in a broad range. Such a hollow fiber type filter has a structure such that many hollow fiber type membrane filters which are generally bundled in an approximately columnar shape are bundled and fixed together with a cylindrical case by potting materials such as an epoxy resin, an urethane resin, or the like.
If a fluid having a different temperature is rapidly poured into a filter having such a structure, a rapid temperature distribution occurs in the radius direction of the cylindrical case in the portion of the potting material, and a large internal stress generates in the portion having a large temperature gradient. When the direction of this stress is in a tensile direction, it may cause a split between the potting material and the case in some occasions. If the split between the potting material and the case occurs, a problem arises such that a liquid leaks from this portion.
The following methods have conventionally been employed in order to prevent the above problem.
(1) Grooves are formed on the inner surface of the case.
(2) An elastic sealing material such as O ring is inserted between the case and the potting material.
(3) Two kinds of the potting materials are used, one being a potting material for imparting a strength, and another being a flexible potting material for preventing the split.
However, the first method has the disadvantages that the potting material cannot completely remove the air in the grooves, and even if the grooves are shaped so as to have a wedge effect, it is not still sufficient to prevent the split. Further, the stress concentrates in the potting materials on the grooves, so that cracks occur in the potting material per se.
The second method has the disadvantage that the sealing material such as O ring is embedded in the potting material, and as a result, the sealing effect cannot sufficiently be exhibited.
The third method has the disadvantage that the casting must be conducted twice, and this is not economically preferred.
Thus, various counterplans have been made so as not to occur the split between the potting material and the case for the hollow fibers when a rapid temperature change is imparted to the hollow fiber type filter, but no satisfactory method has yet been obtained at the present. Since the more the modulus of elasticity of the potting material and the case increases, the more the internal stress generated by the temperature gradient increases, the splitting is liable to occur when materials having a large modulus of elasticity are used. Further, the splitting is also liable to occur where an adhesive strength between the case and the potting material is small. Therefore, in order to prevent the splitting, it is preferred that the case and the potting material have a small modulus of elasticity and are flexible, and that the mutual adhesive strength is large. On the other hand, it is also required for the case and the potting material to have a heat resistance and a pressure resistance. Therefore, a hollow fiber filter which can withstand a large temperature change cannot be obtained unless those antipodal characteristics are simultaneously satisfied.
The tensile stress f (kg/cm.sup.2) generated in a potting material when the potting material receives a pressure P (kg/cm.sup.2) is as follows (cf. Oguri et al., Kikai Sekkeizuhyo Binran, Kyoritsu Shuppan K. K., 1985). EQU f=0.75.times.P.times.(r.sup.2 /t.sup.2) (1)
wherein r is a radius of a plane disc-type potting material (cm), t is a thickness thereof (cm).
For example, assuming that a hollow fiber type filter having substantially the same size as the commercially available cartridge filter having r=35 mm and t=10 mm receives a pressure of 2 kg/cm.sup.2, the maximum tensile stress of 20 kg/cm.sup.2 calculated from the above equation (1) generates in the potting material. Since this pressure corresponds to a heating steam at about 120.degree. C., it is required for the potting material in this filter to have a permissible stress of at least about 25 kg/cm.sup.2 in the temperature range of from room temperature to about 120.degree. C.
Further, when the above-described filter at 20.degree. C. is rapidly heated with a steam at 120.degree. C. from the outside of the case, a temperature gradient of about 100.degree. C. occurs in the interface between the case and the potting material. In this case, if a coefficient of thermal expansion of the case is represented by .alpha., and a modulus of elasticity of the potting material is represented by e (kg/cm.sup.2), a stress .sigma. (kg/cm.sup.2) represented by the following equation generates in the direction of the radius of the potting material at the interface between the potting material and the case by the thermal expansion of the case. EQU .sigma.=100.alpha..times.e (2)
In general, a plastic material is used for the case, and it is considered that the coefficient of thermal expansion .alpha. is about 10.sup.-4, and the modulus of elasticity e of the potting material in the vicinity of room temperature, which comprises an epoxy resin or an urethane resin, each having a heat resistant property, is 5,000 to 10,000 kg/cm.sup.2. Therefore, the stress .sigma. generated at the interface is 50 to 100 kg/cm.sup.2. In other words, the adhesive strength between the potting material and the case must be 50 to 100 kg/cm.sup.2 or more in order to prevent occurrence of the splitting at the interface, but it is not practically easy to achieve such a large adhesive strength with the potting material only.
Further, where the above-described temperature gradient is imparted, a thermal expansion generates in single % order such as in 2% or 5% calculated from the equation(2). And the potting material itself is broken unless the potting material can follow up the elongation. Therefore, the maximum elongation of the potting material must be at lease 10%.